"microsoft does not recommend creating custom master pages or customizing the default master page(...), the reason for this is that if the master page is customized a site collection will not get any fixes or new capabilities that are added to the platform by microsoft"

are you kidding me? that is probably the main reason why people should create custom master pages. Whenever microsoft realeases "any fixes or new capabilities" they could end up ruining the functionality of my site, i'd rather have more control over when and what "new capabilities" are added.

I have been following this tutorial and it has helped a lot! Great one!!!

However facing issues with Mobile pages. So I have a JS which must run on all pages of SharePoint to have control of them. Now, I want it to be executed on mobile pages too but its not being injected to mobile pages because it seems they dont have master page.

I have a master page in SharePoint Online and would like to add a link below the search bar.This master page is at Site Collection level and all subsites inherit this master page.Now if I use the technique shown in this video then does my customization going to be applied in all subsites as well or just root site in the site collection.

Please notice that this video is from 2014 and completely outdated. We do recommend using SharePoint Framework as the extensibility model for SharePoint where possible. Please see the official documentation for more details at http://aka.ms/spdev-docs.

Is this still a valid (recommended) approach to getting javascript into a SharePoint site/page for Office 365 SharePoint users? Some comments here suggest this is outdated but the content editor web part appears dead and gone and I see no other official-sounding means of getting javascript into SharePoint.

Please read first..."Please notice that this video is from 2014 and completely outdated. We do recommend using SharePoint Framework as the extensibility model for SharePoint where possible. Please see the official documentation for more details at http://aka.ms/spdev-docs."